On 2019-09-05, Al Viro viro@zeniv.linux.org.uk wrote:
On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:
+/*
- "memset(p, 0, size)" but for user space buffers. Caller must have already
- checked access_ok(p, size).
- */
+static int __memzero_user(void __user *p, size_t s) +{
- const char zeros[BUFFER_SIZE] = {};
- while (s > 0) {
size_t n = min(s, sizeof(zeros));if (__copy_to_user(p, zeros, n))return -EFAULT;p += n;s -= n;- }
- return 0;
+}
That's called clear_user().
Already switched, I didn't know about clear_user() -- I assumed it would've been called bzero_user() or memzero_user() and didn't find it when looking.
+int copy_struct_to_user(void __user *dst, size_t usize,
const void *src, size_t ksize)+{
- size_t size = min(ksize, usize);
- size_t rest = abs(ksize - usize);
- if (unlikely(usize > PAGE_SIZE))
return -EFAULT;Why?
- } else if (usize > ksize) {
if (__memzero_user(dst + size, rest))return -EFAULT;- }
- /* Copy the interoperable parts of the struct. */
- if (__copy_to_user(dst, src, size))
return -EFAULT;Why not simply clear_user() and copy_to_user()?
I'm not sure I understand what you mean -- are you asking why we need to do memchr_inv(src + size, 0, rest) earlier?
+int copy_struct_from_user(void *dst, size_t ksize,
const void __user *src, size_t usize)+{
- size_t size = min(ksize, usize);
- size_t rest = abs(ksize - usize);
Cute, but... you would be just as well without that 'rest' thing.
I would argue it's harder to mess up using "rest" compared to getting "ksize - usize" and "usize - ksize" mixed up (and it's a bit more readable).
- if (unlikely(usize > PAGE_SIZE))
return -EFAULT;Again, why?
As discussed in a sister thread, I will leave this in the callers (though I would argue callers should always do some kind of sanity check like this).
- if (unlikely(!access_ok(src, usize)))
return -EFAULT;Why not simply copy_from_user() here?
- /* Deal with trailing bytes. */
- if (usize < ksize)
memset(dst + size, 0, rest);- else if (usize > ksize) {
const void __user *addr = src + size;char buffer[BUFFER_SIZE] = {};while (rest > 0) {size_t bufsize = min(rest, sizeof(buffer));if (__copy_from_user(buffer, addr, bufsize))return -EFAULT;if (memchr_inv(buffer, 0, bufsize))return -E2BIG;Frankly, that looks like a candidate for is_all_zeroes_user(). With the loop like above serving as a dumb default. And on badly alighed address it _will_ be dumb. Probably too much so - something like if ((unsigned long)addr & 1) { u8 v; if (get_user(v, (__u8 __user *)addr)) return -EFAULT; if (v) return -E2BIG; addr++; } if ((unsigned long)addr & 2) { u16 v; if (get_user(v, (__u16 __user *)addr)) return -EFAULT; if (v) return -E2BIG; addr +=2; } if ((unsigned long)addr & 4) { u32 v; if (get_user(v, (__u32 __user *)addr)) return -EFAULT; if (v) return -E2BIG; }
<read the rest like you currently do> would be saner, and things like x86 could trivially add an asm variant - it's not hard. Incidentally, memchr_inv() is an overkill in this case...
Why is memchr_inv() overkill?
But yes, breaking this out to an asm-generic is_all_zeroes_user() wouldn't hurt -- and I'll put a cleaned-up version of the alignment handling there too. Should I drop it in asm-generic/uaccess.h, or somewhere else?